The present invention relates to a method for detecting colored foreign particles in a white or light-colored powder and a system therefor. More particularly, the invention relates to a method for detecting colored foreign particles in a white or light-colored powder, such as a powder of a synthetic resin, e.g., polyvinyl chloride, in a continuous process with an object of process control and quality control in order to feed the results of detection back to the production line as well as to a system for practicing the method.
As is well known, synthetic resins such as polyvinyl chloride are widely used as a material for manufacturing various kinds of shaped articles such as sheets, films, decorative boards, containers and so on used for wrapping of food, clothings and the like as well as parts of electric and electronic appliances. It is a recent trend in these applications that the resin powder is desired to contain colored foreign particles as small as possible in number to comply with the demand for shaped articles of higher and higher quality and commercial value. Foreign particles intermixed in the resin powder are very detrimental on the quality of the final products not only in the appearance but also in their practical performance to cause incomplete printing thereon and decrease in the mechanical strengths thereof leading to fissures and cracks formed in the shaped articles.
Accordingly, it is usual in the production plants of such a resin in a powdery form to undertake a step of quality inspection to detect such colored foreign particles among other various items of inspection required before shipping of the product to discard a lot of the product containing such colored foreign particles in excess of a certain tolerable quality control level in number. Conventional methods undertaken for such a purpose are mostly conducted by visual inspection and picking up of the foreign particles by manual works although a proposal has been made for a method by using an instrument such as stroboscope to conduct image analysis while such an instrumental method is practiced only in a limited number of production lines.
Needless to say, the results of a visual inspection, which inherently cannot be very efficient taking a long time, largely depend on the ability of the individuals who are subject to eye fatigue possibly to cause overlooking of the foreign particles so that the method relying on visual inspection is not suitable for a large number of samples or a large quantity of powdery products. Moreover, modern lines for the production of powdery resins are always operated in a closed system so that it is difficult to take samples of the resin powder at as many as desired spots in the production line. Accordingly, the intermixing conditions of foreign particles in the powdery resin product can hardly be under control on the base of the information obtained by the inspection of a limited number of samples taken from the production line.
The instrumental methods by image analysis can solve the problem due to the ability difference of individuals. Conventional optical instruments for inspection, however, have a limited width of the visual field so that the amount of the powder which can be inspected at one time is relatively small providing no solution of the problem in the visual method that the overall throughput of the product inspected by the inspection system is far from satisfactory. This situation is one of the reasons for the retarded prevalence of the instrumental inspecting methods.
In view of the above described situation and problems in the prior art, the inventors have continuedly conducted investigations for the automatization of the inspection system for the detection of colored foreign particles in polyvinyl chloride resin products and previously proposed a method and apparatus for the detection and counting of colored foreign particles and fish eyes in a continuously running film or sheet of a polyvinyl chloride resin by using a transmission-type gas-laser beam scanning detector (see Japanese Patent Kokai 58-108440). This method, however, is not applicable to the resin products in a powdery form because the laser beam has only very low transmissivity through a powder layer and is strongly reflected on the surface of the powder layer so that the results of counting inherently cannot be accurate enough and colored foreign particles alone cannot be counted selectively. In addition, the method is applied to the inspection of a powdery resin only after shaping the powdery resin into a transparent sheet or film so that the material to be inspected is subject to intermixing of foreign materials by chance in the course of shaping the powder into a film or sheet and sometimes scorched particles of the resin per se occurred in the process of shaping resulting in an inaccurate result of inspection.